United States Patent [19]
`Smith
`
`Illllllllllllllllllllllllll|||||llillIlll!llllllllllllllllillllllllllllllll
`USOO5321372A
`[11] Patent Number:
`5,321,372
`[45] Date of Patent:
`Jun. 14, 1994
`
`[54] APPARATUS AND METHOD FOR
`TERMINATING CABLES TO MINIMIZE
`EMISSIONS AND SUSCEPTIBILITY
`.
`_
`[75] Invenwr‘ Rm?“ w- S'mth’ L°S Ah“, ca“-
`[73] Assignee: gynopglcs cog;_mica?ons’ Inc"
`mm am’ a 1 '
`[21] Appl. No.: 1,809
`[22] Filed:
`Jan. 8, 1993
`[51] ‘Int. Cl.5 ............................................. .. H0311 7/00
`[52] US. Cl. .......................... ..
`333/1; 333/12;
`333/22 R; 174/34; 379/416
`[58] Field Of Search ......................... .. 333/ 1, 12, 22 R;
`307/89-91; 174/32-34, 36; 379/398, 416
`References Cited
`
`[56]
`
`5,113,159 5/1992 Adriaensens et al. .............. .. 333/12
`
`FOREIGN PATENT DOCUMENTS
`714544 12/1941 Fed. Re .of German ........ .. 333/1
`79551 6/1981 Japan
`...................
`333/22 R
`Primary Examiner-Paul Gensler
`Attorney, Agent, or Firm-Blakely, Sokoloff, Taylor &
`Zafman
`ABSTRACT
`[57]
`In a computer network having a cable for transmission
`of electrical signals between data terminal equipment
`and a device used to control and route data in the net
`work, an apparatus for terminating the cable to mini
`mize emissions radiating from the cable and susceptibil
`ity of the cable to outside interferences. The cable is
`comprised of unshielded twisted pairs of wires. Each
`twisted pair is individually terminated into a matching
`input impedance. The difference signals being con
`ducted in each twisted pair produces common mode
`currents which sets up standing waves through the
`cable. Consequently, at each end of the cable, the com
`_ 333/12
`m0" mode of the twisted Pairs is terminated into 3 1°“!
`333/1
`having an impedance approximately equal to the com
`.... .. 333/5
`178/63 D mon mode impedance of the twisted pairs. These loads
`effectively dissipate the energy from the standing
`waves, thereby minimizing emissions. Reciprocally,
`
`-
`
`U'S‘ PATENT DOCUMENTS
`1,998,960 4/1935 Kaar .
`2,026,308 12/1935 Ganz -
`33233328
`?rozsley ‘
`3’705’365 12/1972 sgbgret' a1
`3I731I234 5/1973 Collins .... 3.3.1.1.‘:
`4,056,790 11/1977 Pospischil et a1.
`4,697,051 9/1987 Beggs et al. ..... ..
`‘:Zggg;
`153mm‘
`
`’
`
`’
`
`eggs 6 a '
`
`
`
`132223222 24122: 22:11:: 2:31;..- 4,873,393 10/1989 Friesen et al. . . . .
`
`
`
`11113337252153 . . . . . . .. 174/34
`
`5,027,088 6/1991 Shimizu et al. ....................... _. 333/1
`
`17 Claims, 3 Drawing Sheets
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`US. Patent
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`-
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`June 14, 1994
`
`Sheet 1 of 3
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`5,321,372
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`102
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`101E11D11U
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`FIGURE 1
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`US. Patent
`
`June 14, 1994
`
`Sheet 2 of 3
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`5,321,372
`
`Figure 2
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`202W
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`//
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`Figure 3
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`US. Patent
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`June 14, 1994
`
`Sheet 3 of 3
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`5,321,372
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`Flgure 4 g
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`1
`
`APPARATUS AND METHOD FOR TERMINATING
`CABLES TO MINIMIZE EMISSIONS AND
`SUSCEPTIBILITY
`
`35
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`5,321,372
`2
`can lead to signal errors. Also, signals being conducted
`on these twisted pairs suffer from attenuation.
`Increasing the signal strength addresses both these
`issues, but it also produces higher levels of emissions
`from the twisted pair wires. Government regulations
`mandate that emissions be limited to a particular level in
`order to minimize interferences with other apparatus.
`Thus, the signal strength for twisted pairs is governed
`by the standards set forth by the FCC. Sometimes, a
`shielding made of wire mesh or foil having a high per
`meability is used to surround the twisted pair wiring.
`One such implementation wherein four twisted shielded
`pairs for conducting differential signals, a 12 volt power
`line, and a master shielding and insulation layer encom
`passing all the wires is known as an Attachment Unit
`Interface (AUl) cable. For broadband and many base
`band systems, an alternative to twisted pairs is coaxial
`cables. Coaxial cables have a single center conductor
`surrounded by an insulator, which is then enclosed by a
`metal shield such as a wire mesh or foil. Coaxial cables
`can handle greater bandwidths and are less susceptible
`to outside noise. However, coaxial cables are typically
`more expensive than twisted pairs.
`Fiber optic cables are also being implemented in com
`puter networks. Fiber optic cables are generally im
`mune to electrical noise and are capable of handling
`very high bandwidth and transmission speed. However,
`splicing and tapping ?ber optic cables is an expensive
`and dif?cult process. Moreover, ?ber optic cables are
`very expensive. Hence, ?ber optic cables are typically
`applied for long distances and heavy traf?c.
`Choosing among these various different media is a
`matter of tradeoffs. For most small local area networks
`(LANs), cost is of paramount concern. Signi?cant costs
`are incurred not only for purchasing the medium itself
`but also for physically routing the medium to each of
`the various terminals. Costs could be greatly reduced if
`the network could take advantage of an already existing
`medium. One such medium is the telephone lines al
`ready existing and installed in virtually all of?ce build
`ings, factories, and homes. These same telephone lines
`comprising twisted pair wiring can be used to conduct
`digital signals for computers rather than analog signals
`for, phone systems. The disadvantage with using tele
`phone wiring is that they are limited to short run -
`lengths because of the susceptibility and emissions prob
`lems inherent to all twisted pair wiring. In some cases,
`longer lengths are required than can be met by tele
`phone lines. Consequently, more expensive medium
`have to be bought and installed in these circumstances.
`Therefore, there is a need in the prior art for an appa
`ratus and method for minimizing the susceptibility and
`emissions of twisted pair wiring in order that longer
`lengths can be implemented to conduct digital signals in
`a computer network. It would also be highly preferable
`for the apparatus and method to be inexpensive and easy
`to implement.
`SUMMARY OF THE INVENTION
`In response to the shortcomings associated with prior
`art twisted pair cables used in the transmission of digital
`signals in a computer network, the present invention
`minimizes emissions radiating from such a cable and the
`susceptibility of signals on the cable to outside interfer
`ences. This is accomplished by terminating each twisted
`pair into a matching input impedance of either a data
`terminal equipment, such as a personal computer, or a
`
`FIELD OF THE INVENTION
`The present invention relates to the ?eld of transmis
`sion lines. More particularly, the present invention per
`tains to an apparatus and method for terminating
`twisted pair cables to minimize emissions and suscepti
`bility.
`BACKGROUND OF THE INVENTION
`In the ?eld of computer systems, computer networks
`have enjoyed increasing popularity because of their
`versatility, ?exibility, and economy. In contrast to pow
`erful, centralized and costly “mainframes,” wherein
`processing is routed to and performed by the mainframe
`computer, a computer network typically distributes
`processing to various autonomous computers. These
`individual computers are coupled together so that each
`computer in the network can communicate with other
`computers connected to the network. This arrangement
`allows end users to work on a joint project on separate
`computers by sharing ?les and swapping information
`over the network. Furthermore, application programs,
`databases, and peripheral hardware such as printers,
`disk drives, etc., can be shared amongst the various end
`users. Thereby, information and computing resources
`can readily be pooled amongst individuals and work
`groups. In addition, reliability is improved because a
`breakdown of one computer on the network does not
`result in shutting down or crashing the entire computer
`network. The other parts of the computer network can
`continue to function while the particular failed compo
`nent is being repaired. Moreover, it is a relatively simple
`procedure to upgrade an existing computer network by
`hooking up additional computers and network control
`equipment on an as-needed basis.
`Typically, a computer network is comprised of a
`number of data terminal equipment (DTE) which are
`coupled together by transmission lines. DTEs include
`personal computers, workstations, an input terminal,
`etc. Each DTE has a transceiver which couples the
`DTE to the transmission line. The transceiver transmits
`data onto and receives data from the transmission lines
`as well as performing any necessary signal conversions.
`Numerous transmission lines conduct digital bits of data
`between the DTEs. Often, the transmission lines are
`50
`connected to a hub or concentrator which is used to
`route and manage signal traf?c on the network. To
`expand a computer network, repeaters are used to copy
`and forward bits of data from one network to another;
`bridges are implemented to interface networks having
`different protocols; and routers are used for ef?ciently
`directing data packets to their ?nal destination.
`Presently, there exists many different types of media
`which can be used for propagating the transmission of
`data between the various devices of a computer net
`work. The different types of media include unshielded
`twisted pair wiring, shielded twisted pair wiring, coax
`ial cables, and ?ber optic cables. Traditionally, electri
`cal communications were conducted over two insulated
`copper wires twisted together, hence the term twisted
`pair wiring. However, twisted pair wiring is susceptible
`to outside interference which introduces distortions.
`The distortions caused by outside sources increase with
`longer distances and higher data rates. These distortions
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`device used to control and route data through the com
`puter network.
`The difference signals being conducted in each
`twisted pair produces common mode currents. In turn,
`these common mode currents generate standing waves
`through the cable. If left unterminated, the energy from
`the standing waves radiate to the surroundings. Conse
`quently, the present invention terminates the common
`mode of the twisted pairs into a load having an impe
`dance approximately equal to the common mode impe
`dance of the twisted pairs. The cable is so terminated at
`both ends. These loads effectively dissipate the energy
`from the standing waves, thereby minimizing emissions.
`Reciprocally, susceptibility is also minimized.
`In the currently preferred embodiment of the present
`and 5
`invention, resistors are used as loads. The common
`mode impedance of the twisted pairs are measured, and
`an equivalent resistor value is implemented. Transform
`ers are coupled across the individual twisted pairs. The
`resistors are coupled to the center taps of the transform
`ers. This arrangement minimizes the impact of the com
`mon mode loads on the difference signals being trans
`mitted on the twisted pairs while effectively providing
`a common mode termination. As a result, the energy
`associated with the standing waves is dissipated in the
`form of heat in the resistors rather than as RF energy to
`its surroundings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The present invention is illustrated by way of exam
`ple, and not by way of limitation, in the ?gures of the
`accompanying drawings and in which like reference
`numerals refer to similar elements and in which:
`FIG. 1 illustrates a computer network which the
`present invention may be practiced upon.
`FIG. 2 shows an unshielded twisted pair cable.
`FIG. 3 shows the currently preferred embodiment of
`the present invention as applied to an unshielded cable
`having four twisted pair wires.
`FIG. 4 shows another view of the currently preferred
`embodiment of the present invention as applied to an
`unshielded twisted pair cable.
`DETAILED DESCRIPTION
`An apparatus and method for minimizing susceptibil
`ity and emissions in twisted pair wiring is described. In
`the following description, for purposes of explanation,
`numerous speci?c details are set forth, such as resistor
`values, transformer types, cable classi?cations, network
`components, etc., in order to provide a thorough under
`standing of the present invention. It will be obvious,
`however, to one skilled in the art that these speci?c
`details need not be used to practice the present inven
`tion. In other instances, well-known structures and cir
`cuits have not been shown in detail in order to avoid
`unnecessarily obscuring the present invention.
`Referring to FIG. 1, a typical computer network
`system upon which the present invention may be uti
`lized is shown. A number of users access the computer
`network through data terminal equipment (DTE) 101.
`DTE 101 can include personal computers, worksta
`tions, portable computers, minicomputers, dumb termi
`nals, etc. DTE 101 are coupled to concentrators 102 by
`unshielded twisted pair cables 103. Additionally, a ?le
`server 104 can be coupled to a concentrator 102 so that
`users can share stored ?les, data bases, and application
`programs. Concentrators 102 act as focal points for
`managing and routing network data communications.
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`5,321,372
`4
`Concentrators 102 can be coupled together. A repeater
`105 can be used to amplify attenuated signals. A bridge
`106 can be used to couple together two concentrators
`which service network segments having different pro
`tocols (e.g., an Ethemet-to-Token Ring bridge). Bridge
`106 listens to data transmissions, examines each data
`packet’s destination, makes any necessary conversions,
`and discriminately forwards each data packet. Some
`times a network management station (not shown) is
`used to collect, monitor, display, and control various
`aspects of the computer network.
`The computer network of FIG. 1 can be expanded to
`encompass hundreds of users by adding extra DTEs,
`concentrators, repeaters, and bridges. Furthermore, a
`router can be used to interface networks having differ
`ent network layers such as Token Bus versus X.25. In
`addition, various parts of the computer network can be
`con?gured any number of different topologies (e.g.,
`star, ring, tree, et.) and different protocols such as
`Ethernet, token ring, token bus, etc. can be utilized. It
`should be noted that the present invention applies to
`any type of computer network con?guration, regardless
`of size and make, which utilizes twisted pair wires for
`data communications.
`FIG. 2 shows a typical unshielded twisted pair cable
`201. Cable 201 comprises an outer plastic sheath 202,
`and four pairs of insulated copper wires 203-206. Each
`pair is capable of conducting a differential electrical
`signal. In a differential signal scheme, the data is inde
`pendent of the individual signal levels. Instead, informa
`tion is conveyed as the difference of the voltage levels
`being conducted between the two wires. A differential
`ampli?er is used to amplify the received difference
`voltage between the signals on the two wires. When the
`signals on the two wires both change levels together, it
`is referred to as common-mode change. A differential
`change is known as normal mode. A differential ampli
`?er having a high common-mode rejection ratio (i.e.,
`the ratio of response for a normal-mode signal to the
`response for a common-mode signal of the same ampli
`tude) are used in those situations wherein weak signals
`are susceptible to noise and crosstalk.
`One end of the twisted pair wires is coupled to a
`DTE, while the other end is coupled to a concentrator
`or some other equivalent control/routing mechanism.
`Typically, two twisted pairs are used for providing
`communication between a DTE and a concentrator.
`One twisted pair is used to transmit digital data. The
`second twisted pair is used to receive digital data. In
`order to reduce susceptibility and emissions, these sig
`nals are terminated into matched loads. It is highly
`desirable to match the load to the characteristic impe
`dance of the twisted pair because for high frequency
`and short rise-time signals, a transmission line termi
`nated with a load equal to its characteristic impedance
`transfers the electrical signal without re?ection. In so
`doing, all the power in the signal is transferred to the
`load. Furthermore, the impedance looking into such a
`terminated line is equivalent to its characteristic impe
`dance, irregardless of its frequency. In one example,
`both the DTE and the concentrator have 100 ohm input
`impedances. Terminating each twisted pair thusly, aids
`in reducing the problems of susceptibility and emissions.
`However, it has been discovered in the present inven
`tion that imbalances between adjacent twisted pairs of
`wires running the length of an unshielded twisted pair
`cable causes common-mode currents to be transferred
`from the differential currents in each twisted pair aris
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`node 310 could be, but does not necessarily have to be
`ing from the signals being conducted. This common
`grounded. Transformers 311-314 are 1:1 center tap
`mode coupling effect results in standing waves in the
`transformers. This same transformer and resistor ar
`longitudinal mode. A standing wave is de?ned in which
`rangement is implemented for the other end of cable 301
`for any component of the ?eld, the ratio of its instanta
`as well.
`neous value at one point to that at any other point does
`The twisted pairs from the transformers 311-314 are
`not vary over time. This resulting standing wave is
`coupled to the DTE/concentrator. By implementing
`longitudinal in that the direction of its displacement at
`the transformer and resistor arrangement, the energy in
`each point of the twisted pair is the same as the direction
`the standing waves caused by the common-mode cur
`of the propagation. In the prior art. The energy of these
`rents are dissipated in the 75 ohm resistors 306-309,
`standing waves are dissipated by emanating from the
`with negligible or no impact on the data being transmit
`unshielded twisted pair cable. In other words, the un
`ted through the individual twisted pairs 302-305.
`shielded twisted cable forms a common-mode radiator.
`In an alternative embodiment, chokes can be used
`In the present invention, this common-mode effect is
`instead of resistors. In other embodiments, equivalent
`minimized by terminating the common-mode of the
`circuits can be implemented as loads so long as they
`differential twisted pairs into balanced loads. In the
`have an impedance approximately matching that of the
`currently preferred embodiment of the present inven
`common-mode impedance. For example, two 145 Q
`tion, this is accomplished by determining the impedance
`resistors can be substituted for the four 75 Q. resistors.
`between the twisted pairs. An impedance meter can be
`One of these resistors is coupled across the center taps
`used to measure this common-mode impedance. For a
`of transformers 312 and 314, while the other resistor is
`standard Category Five unshielded twisted pair cable,
`coupled across the center taps of transformers 311 and
`the common-mode impedance is approximately 145
`313.
`ohms. By terminating the two common-mode differen
`FIG. 4 is another illustration of the currently pre
`tial pairs of an unshielded twisted pair cable into 145
`ferred embodiment of the present invention. One end of
`ohm loads, the standing waves are absorbed by the
`the unshielded twisted pair cable 401 is coupled to a
`loads. The end result is a ?at cable with minimal stand
`DTE, such as to the input ?lter or transceiver of a
`ing waves and greatly minimized emissions from the
`personal computer. The other end of cable 401 is cou
`common-mode radiator.
`pled to a concentrator or its like. Cable 401 is comprised
`In summary, the energy in the standing waves is
`of four twisted pair wires 402-405. It can be seen that
`dissipated in the form of heat in the loads rather than
`the transformer and resistor circuit is implemented at
`radio frequency (RF) energy to it’s surroundings. Ac
`both ends of cable 401. Transformers 406-413 in combi
`cording to the reciprocity theorem, if an electromag
`nation with resistors 414-421 effectively absorb the
`netic force at one point in a network produces a current
`standing waves produced by the common-mode cur
`at a second point in the network, then the same voltage
`rents of twisted pairs 402-405. Implementing such a
`acting at the second point will produce the same current
`transformer and resistor combination can result in up
`at the ?rst point. Consequently, applied to the instant
`wards of approximately 10 dB improvement in emis
`case, the amount of susceptibility to interfering external
`sions and susceptibility.
`electromagnetic ?elds should, likewise, be minimized
`Although the above description was in relation to an
`by the same amount as the reduction in emissions pro
`unshielded twisted pair cable, the present invention can
`duced by the present invention.
`equally be applied to shielded twisted pair cables. The
`In the currently preferred embodiment of the present
`shielding would further reduce emissions and suscepti
`invention, a combination of transformers and resistors
`bility. In addition, the present invention is not limited to
`are implemented as loads for terminating the common
`cables having four twisted pairs in a quad con?guration.
`mode differential pairs as shown in FIG. 3. An un
`The present invention can be applied to any combina
`shielded twisted pair cable 301 having four twisted pairs
`tion of twisted pair wires of two or more. The present
`of wires 302-305 is shown. Each of the twisted pairs is
`invention is equally applicable to twisted pairs running
`terminated into an input impedance of 100 ohms to
`in straight parallel lines as well as twisted pairs which
`match their characteristic impedance. In addition, ac
`are then twisted amongst themselves, so long as symme
`cording to the present invention, four 75 ohm resistors
`try is maintained. Moreover, the present invention can
`306-309 are used as loads for terminating the common
`be practiced upon untwisted wires, such as untwisted
`mode differential pairs. It should be noted that the resis
`telephone wires. Furthermore, terminating the com
`tance being implemented depends on the common
`mon-mode according to the present invention at just
`mode impedance of that particular type of cable.
`one of the ends of the cable is an improvement over no
`Resistor 306 is coupled at one end to the center tap of
`such terminations at all.
`transformer 314. The windings of transformer 314 are
`In the currently preferred embodiment of the present
`coupled to twisted pair 303. The other end of resistor
`invention, the transformers and resistors can be imple
`306 is coupled to node 310. Resistor 307 is coupled at
`mented as part of the transceiver found in the DTE and
`one end to the center tap of transformer 312. The wind
`concentrator. Alternatively, the transformers and resis
`ings of transformer 312 are coupled to the twisted pair
`tors can be implemented within an external media ?lter.
`302. The other end of resistor 307 is coupled to node
`This ?lter normally conditions signals prior to transmis
`310. Likewise, one end of resistor 308 is coupled to the
`sion on the cable.
`center tap of transformer 313. The windings of trans
`Thus, an apparatus and method for minimizing emis
`former 313 are coupled to the twisted pair 304. The
`sions in a twisted pair cable is described.
`other end of resistor 308 is coupled to node 310. Resis
`What is claimed is:
`tor 309 is coupled at one end to the center tap of trans
`1. A communications system for transmission of an
`former 311. The windings of transformer 311 are cou
`electrical signal having a means for generating said
`pled to the twisted pair 305. The other end of resistor
`electrical signal, a means for transmitting said electrical
`309 is coupled to node 310. Thus, all four resistors
`signal, a means for receiving a transmitted electrical
`306-309 are coupled at one point, node 310. Note that
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`signal, and a means for conducting said transmitted
`electrical signal from said generating means to said
`receiving means, said conducting means comprising:
`a ?rst pair of twisted wires for conducting a signal
`which is terminated into an input impedance
`matched to said pair of twisted wires;
`a second pair of twisted wires for conducting a sec
`ond signal which is terminated into an input impe
`dance matched to said second pair of wires;
`a means for terminating a common mode of said ?rst
`pair of wires and said second pair of wires into a
`load having an impedance approximately equal to a
`common mode impedance of said ?rst pair of wires
`and said second pair of wires, wherein said com
`mon mode of said ?rst pair of wires and said second
`pair of wires are terminated into a ?rst load at one
`end of said wires and into a second load at another
`end of said wires, said ?rst load and said second
`load comprising resistors wherein susceptibility
`and emissions of said conducting means are mini
`mized.
`2. The communications system of claim 1, wherein
`said conducting means further comprises:
`a ?rst transformer coupled to said ?rst pair of wires;
`a second transformer coupled to said second pair of
`wires, wherein said ?rst load is coupled to a ?rst
`center tap of said ?rst transformer and a second
`center tap of said second transformer.
`3. The communications system of claim 2, wherein
`said conducting means further comprises:
`a third pair of twisted wires for conducting a third
`signal which is terminated into an input impedance
`matched to said third pair of wires;
`a fourth pair of twisted wires for conducting a fourth
`signal which is terminated into an input impedance
`matched to said fourth pair of wires;
`a second means for terminating a common mode of
`said third pair of wires and said fourth pair of wires
`into a second load having an impedance approxi
`mately equal to a common mode impedance of said
`third pair of wires and said fourth pair of wires.
`4. The communications system of claim 3, wherein
`said ?rst signal, said second signal, said third signal, and
`said fourth signal are difference signals.
`5. The communications system of claim 4, wherein
`said ?rst load and said second load reside in said trans
`mitting means and said receiving means.
`6. The communications system of claim 5, wherein
`said ?rst load and said second load reside in a ?ltering
`means for conditioning said ?rst signal.
`7. The communications system of claim 5, wherein
`said conducting means further comprises metallic
`shielding surrounding said ?rst, second, third, and
`fourth pairs of twisted wires.
`8. An apparatus for minimizing susceptibility and
`emissions of a cable having at least a ?rst insulated
`conductor, a second insulated conductor, a third insu
`lated conductor, and a fourth insulated conductor for
`transmission of electrical signals from a source to a
`destination, wherein said ?rst conductor and said sec
`ond conductor are terminated with a matching impe
`dance and said third conductor and said fourth conduc
`tor are terminated with said matching impedance, said
`apparatus comprising:
`a ?rst load comprising a ?rst resistor coupled to said
`?rst, second, third, and fourth conductors for ab
`sorbing energy from standing waves produced by
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`common-mode currents induced between said ?rst,
`second, third, and fourth conductors;
`a second load comprising a second resistor placed at
`an opposing end of said cable, said ?rst load and
`said second load having an impedance approxi
`mately equal to a common mode impedance of said
`?rst, second, third, and fourth conductors.
`9. The apparatus of claim 8 further comprising:
`a ?rst transformer coupled to said ?rst conductor and
`said second conductor;
`a second transformer coupled to said third conductor
`and said fourth conductor, wherein said ?rst resis
`tor is coupled to a center tap of said ?rst trans
`former and a center tap of said second transformer.
`10. The apparatus of claim 9 further comprising:
`a third transformer coupled to said ?rst conductor
`and said second conductor;
`a fourth transformer coupled to said third conductor
`and said fourth conductor, wherein said second
`resistor is coupled to a center tap of said third
`transformer and a center tap of said fourth trans
`former and wherein said ?rst transformer and said
`second transformer are placed at said opposing
`ends of said cable from said third transformer and
`said fourth transformer.
`11. The apparatus of claim 10, wherein said ?rst resis
`tor, said ?rst transformer, and said second transformer
`reside within said data terminal equipment.
`12. The apparatus of claim 10, wherein said ?rst resis
`tor, said ?rst transformer, and said second transformer
`are placed within an apparatus for ?ltering said electri
`cal signal.
`13. In a computer network having a plurality of digi
`tal terminal equipment for providing a plurality of users
`access to said computer network, a means for control
`ling data between said digital terminal equipment, and a
`conductor comprising two pairs of wires for conducting
`electrical signals between said digital terminal equip
`ment and said controlling means, an apparatus for mini
`mizing emissions from said conductor comprising:
`a means for terminating said two pairs of wires at one
`end to said digital terminal equipment and at the
`other end to said controlling and routing means,
`, wherein said digital terminal equipment and said
`controlling and routing means have input imped- .
`ances approximately equal to a characteristic impe
`dance of said two pairs of wires;
`a means for terminating a common mode of said two
`pairs of wires with a ?rst load comprising a ?rst
`resistor having an impedance approximately equal
`to a common mode impedance of said two pairs of
`wires, wherein common mode energy is dissipated
`in said load rather than emanating from said con
`
`45
`
`ductor;
`_
`a means for terminating said common mode with a
`second load comprising a second resistor also hav
`ing an impedance approximately equal to said com
`mon mode impedance, wherein said loads are dis
`posed near opposing ends of said conductor.
`14. The apparatus of claim 13, wherein said conduc
`tor further comprises a third pair and a fourth pair of
`wires for conducting electrical signals between said
`digital terminal equipment and said controlling means,
`wherein each pair of wires are coupled to a transformer
`and a resistor is coupled to a center tap of each trans
`former, each resistor also being coupled to a common
`node.
`
`Juniper Ex 1029 - 8
`Juniper v Chrimar
`IPR2016-01397
`
`

`

`5,321,372
`10
`determining a common mode impedance of said two
`pairs of twisted wires;
`terminating a common mode of said two pairs of
`twisted wires with a resistive load having an impe
`dance approximately equal to said common mode
`impedance of said two pairs of twisted wires,
`wherein common mode energy is dissipated in said
`resistive load rather than emanating from said con
`ductor;
`terminating said common mode with another resis
`tive load also having an impedance approximately
`equal to said common mode impedance; and
`disposing said resistive loads near opposing ends of
`said conductor.
`17. The apparatus of claim 16 further comprising the
`step of implementing transformers for coupling said
`resistive loads to said pairs of wires to minimize impact
`on said electrical signals by said loads.
`# i i # Q
`
`15
`
`15. The apparatus of claim 14, wherein said resistors
`are approximately 75 ohms.
`16. In a computer network having a plurality of digi
`tal terminal equipment for providing a plurality of users
`access to said computer network, a means for control
`ling and routing data between said digital terminal
`equipment, and a conductor comprising at least two
`pairs of twisted wires for conducting electrical signals
`between said digital terminal equipment and said con
`trolling and routing means, a method for minimizing
`emissions from said conductor comprising the steps of:
`terminating said two pairs of twisted wires at one end
`to said digital terminal equipment and at the other
`end to said control